This application is related to commonly-owned, co-pending U.S. patent application No. 07/412,744, filed Sept. 26, 1989, by the inventor and with a similar title.
The present invention relates to digital line subscriber systems, and it relates, more particularly, to digital signal equalization for compensating of distortion.
Circuit arrangements of this type are already known (e.g., NTG Technical Reports 88, "Approaches to the Integrated Communication Network," pages 50 to 57), for the purpose of equalizing digital signals that have been transmitted by means of an analog transmission process and have therefore been subjected to distortion products both of precursors of the subsequent digital signals and postcursors of the received digital signals. In particular, strong linear distortions occur in the digital subscriber lines of the ISDN (Integrated Services Digital Network) which must be eliminated by appropriate regeneration methods in the digital signal receiving equipment. The distortions caused by precursors are generally minor. For example, a transverse filter having two to four filter coefficients is sufficient for equalization. On the other hand, distortions that are produced by postcursors are very large. They can be compensated, for example, by a an equalizer connected upstream of the decision feedback equalizer. The number of typical equalizer coefficients in this case lies between 20 and 40. However, in the case of digital signals with higher values, such as those coded according to a 2B1Q line code, convergence and instability problems may occur in regenerators of this kind when the distortions are severe.